feat(df): adding fillna with value method

modules/underdog-dataframe/src/main/groovy/underdog/DataFrame.groovy

@@ -12,6 +12,24 @@ import java.util.function.Function
  * @since 0.1.0
  */
 interface DataFrame extends Columnar {
+
+    /**
+     * Make a copy of this object’s indices and data.
+     *
+     * @return a new instance of {@link DataFrame}
+     * @since 0.1.0
+     */
+    DataFrame copy()
+
+    /**
+     * Fill NA/NaN values using the specified value passed as parameter
+     *
+     * @param value the value to replace the NA/Nan values with
+     * @return the dataframe with replaced values
+     * @since 0.1.0
+     */
+    DataFrame fillna(Object value)
+
     /**
      * The transpose of the DataFrame.
      *

modules/underdog-dataframe/src/main/groovy/underdog/Series.groovy

@@ -84,6 +84,15 @@ interface Series extends Columnar, Iterable {
      */
     Series dropna()
 
+    /**
+     * Fill NA/NaN values using the specified value
+     *
+     * @param value the value to replace the NA/Nan values with
+     * @return the series instance with replaced values
+     * @since 0.1.0
+     */
+    Series fillna(Object value)
+
     /**
      * @param index
      * @return

modules/underdog-dataframe/src/main/groovy/underdog/impl/TSDataFrame.groovy

@@ -76,6 +76,26 @@ class TSDataFrame implements DataFrame {
         return new TSDataFrame(Table.create(dataFrameName, columns))
     }
 
+    @Override
+    DataFrame copy() {
+        return new TSDataFrame(table.copy())
+    }
+
+    @Override
+    DataFrame fillna(Object o) {
+        Table copied = table.copy()
+        copied.columns().each {
+            if (it.type() == ColumnType.DOUBLE && o.toString().isNumber() && o.toString().isLong()) {
+                it.setMissingTo(o.toString().toDouble())
+            } else if (it.type() == ColumnType.STRING) {
+                it.setMissingTo(o.toString())
+            } else {
+                it.setMissingTo(o)
+            }
+        }
+        return new TSDataFrame(copied)
+    }
+
     @Override
     DataFrame getT() {
         return new TSDataFrame(this.table.transpose())
@@ -487,7 +507,7 @@ class TSDataFrame implements DataFrame {
     @NamedVariant
     Series min(TypeAxis axisType) {
         return null
-min    }
+    }
 
     @Override
     DataFrame minus(Series series) {

modules/underdog-dataframe/src/main/groovy/underdog/impl/TSSeries.groovy

@@ -206,6 +206,11 @@ class TSSeries implements Series {
         return new TSSeries(column.removeMissing())
     }
 
+    @Override
+    Series fillna(Object o) {
+        return new TSSeries(column.setMissingTo(o))
+    }
+
     @Override
     Series lag(int index) {
         return new TSSeries(this.column.lag(index))

modules/underdog-dataframe/src/test/groovy/underdog/DataFrameSpec.groovy

@@ -480,4 +480,38 @@ class DataFrameSpec extends BaseSpec {
         columns == ['id', 'weight']
         weights == [11, 22, 303, 604, 1005] as int[]
     }
+
+    def "[DataFrame/fillna]: fixed value all numeric series"() {
+        setup:
+        def df = [
+            A: [1, 2, null, 3, 4],
+            B: [1.0, 2.0, 3.0, null, 4.0],
+            C: ((1..4).collect { new BigDecimal(it) } + [null])
+        ].toDataFrame("coercing")
+
+        when:
+        df = df.fillna(-1)
+
+        then:
+        df['A'].toList() == [1, 2, -1, 3, 4]
+        df['B'].toList() == [1.0, 2.0, 3.0, -1, 4.0]
+        df['C'].toList() == [1.0, 2.0, 3.0, 4.0, -1.0]
+    }
+
+    def "[DataFrame/fillna]: fixed value all mixed series"() {
+        setup:
+        def df = [
+                A: ('a'..'c') + [null],
+                B: [1.0, 2.0, null, 4.0],
+                C: (1..3) + [null]
+        ].toDataFrame("coercing")
+
+        when:
+        df = df.fillna(-1)
+
+        then:
+        df['A'].toList() == ['a', 'b', 'c', '-1']
+        df['B'].toList() == [1.0, 2.0, -1, 4.0]
+        df['C'].toList() == [1, 2, 3, -1.0]
+    }
 }